Let's do it step by step:
Now you have a single image in your list, numbered
 (or also
Now you have two images
[-2] (original image) and
[-1] (result of the pencil portrait filter).
Using this particular form of the
-blend command modifies only the image
[-1]. Here, it replaces your second image
 (or also
[-1]) by the blending between the original image and the pencil portrait image.
You still have two images in your list at this point, as image
[-2]) has not been modified here.
-reverse) will change the order of your images. Now image
[-2]) is the blending previously computed, and image
[-1]) is the original image, which has been untouched since the beginning of the process.
Here, I use the second form of the command
-blend, which blend selected images together. Here the selection is not written, so it assumes it works on the whole set of image, i.e. command selection is
[-2,-1]. We could have written
-blend[-2,-1] overlay to get the same result. With this form of the
-blend command, it is assumed the first image is the background layer, and the second image is the foreground layer, so for blending in overlay mode here, that's precisely the order I'd like to have (original image as the top layer).
Not sure it helps a lot, I'm so used to write G'MIC command lines that I often forget this is probably not trivial to understand, sorry about that